The present invention relates generally to endoluminal stents, covered stents and stent-grafts designed for delivery into an anatomical passageway using minimally invasive techniques, such as percutaneous intravascular delivery using a delivery catheter passed over a guidewire. More particularly, the present invention relates to endoluminal stents having a scaffold structure and structural geometry which is particularly well-suited for providing physiologically acceptable radial or hoop strength and longitudinal flexibility, while also presenting a luminal surface thereof which presents less obstruction to longitudinal shear forces during fluid flow across the luminal surface of the inventive device while maximizing fatigue life and corrosion resistance. Additionally, the inventive endoluminal stent is characterized by a geometry that uniquely has a negative coefficient of longitudinal foreshortening upon radial expansion. Thus, a unique aspect of the inventive endoluminal stent is that it elongates upon radial expansion.
Endoluminal stents are generally tubular scaffolds fabricated from implantable biocompatible materials. Stents have a generally tubular geometry characterized by a central lumen, a longitudinal axis, a circumferential axis and a radial axis. Conventional endoluminal stents fall within three general classifications: balloon expandable, self-expanding and shape-memory. Balloon expandable stents require mechanical intervention, such as by using a balloon catheter, to apply a positive pressure radially outward from a central lumen of the stent to mechanically deform the stent and urge it to a larger diameter. Self-expanding stents utilize inherent material mechanical properties of the stent material to expand the stent. Typically, self-expanding stents are fabricated of materials that rebound when a positive pressure is exerted against the material. Self-expanding stents are fabricated such that their zero-stress configuration conforms to the second larger diameter. The self-expanding stents are drawn down to the first smaller diameter and constrained within a delivery catheter for endoluminal delivery. Removal of the constraint releases the constraining pressure and the self-expanding stent, under its own mechanical properties, rebounds to the second larger diameter. Finally, shape-memory stents rely upon unique alloys that exhibit shape memory under certain thermal conditions. Conventional shape-memory stents are typically nickel-titanium alloys known generically as nitinol, which have a transition phase at or near normal body temperature, i.e., 37 degrees Centigrade.
The prior art is replete with various stent designs across all stent classifications. One of the difficulties with many conventional stent designs arises due to the conflicting criteria between the desired properties of circumferential or hoop strength of the stent, longitudinal or column strength, longitudinal flexibility, fish-scaling of individual structural members of the stent, fatigue life, corrosion resistance, corrosion fatigue, hemodynamics, radioopacity and biocompatibility and the capability of passing the stent through an already implanted stent. Typically, stents that are designed to optimize for hoop strength typically sacrifice either column strength and/or longitudinal flexibility, while stents that are designed to optimize for column strength often compromise longitudinal flexibility and/or hoop strength.
Most conventional stents exhibit longitudinal foreshortening upon radial expansion of the stent. Longitudinal foreshortening is a well-known property that results from the geometric deformation of the stent's structural members as the stent radially expands from a contracted state to a diametrically expanded state. Several prior art stents have been invented that claim a lack of appreciable foreshortening of the stent as a novel feature of the stent. Heretofore, however, a stent that longitudinally elongates upon radial expansion from a contracted state to a diametrically expanded state is unknown in the art.
It has been found desirable to devise an endoluminal stent which employs a series of first and interconnecting members arrayed in geometrical patterns which achieve a balance between hoop strength, column strength and longitudinal flexibility of the endoluminal stent. Many conventional stents employ a series of circumferential structural elements and longitudinal structural elements of varying configurations. A large number of conventional stents utilize circumferential structural elements configured into a serpentine configuration or a zig-zag configuration. The reason underlying this configuration is the need for radial expansion of the stent. Of these conventional stents which employ serpentine or zig-zag circumferential structural elements, many also employ longitudinal structural elements which join adjacent circumferential structural elements and provide a modicum of longitudinal or column strength while retaining longitudinal flexibility of the device. Additionally, many conventional stents require welds to join mating surfaces of the stent.
Heretofore, however, the art has not devised a unibody stent structural element geometry which achieves a balance between hoop strength, column strength and longitudinal flexibility, degree of longitudinal foreshortening, circumferential strength or hoop strength of the stent, longitudinal strength or column strength, longitudinal flexibility, fish-scaling of individual structural members of the stent, fatigue life, corrosion resistance, corrosion fatigue, hemodynamics, radioopacity, biocompatibility and the capability of passing the stent through an already implanted stent. The term “fish-scaling” is used in the art and herein to describe a condition where some stent structural elements extend beyond the circumferential plane of the stent during either radial expansion, implantation or while passing the stent through a bend in the vasculature. Those of ordinary skill in the art understand that fish-scaling of stent structural elements may cause the stent to impinge or snag upon the anatomical tissue either during endoluminal delivery or after implantation. The term “unibody” as used herein is intended to mean a stent that is fabricated without the use of welds and as an integral body of material.
The inventive endoluminal stent may be, but is not necessarily, fabricated by vapor deposition techniques. Vapor deposition fabrication of the inventive stents offers many advantages, including, without limitation, the ability to fabricate stents of complex geometries, the ability to control fatigue life, corrosion resistance, corrosion fatigue, bulk and surface material properties, and the ability to vary the transverse profiles, Z-axis thickness and X-Y-axis surface area of the stent's structural elements in manners that affect the longitudinal flexibility, hoop strength of the stent and radial expansion profiles.